In an electronic device, for example, in a laser printer, a paper feed roller (an element to be driven) provided in a main body case is coupled to a motor, and the paper feed roller is driven and rotated by the motor, and the paper is sent to a specified position.
This motor is generally a brushless DC motor. The motor includes a stator having a plurality of magnetic poles disposed on its circumference at first prescribed interval, and a rotor disposed on the circumference of this stator. On the inner circumference of the rotor, permanent magnets magnetized to different polarities are disposed at second prescribed intervals.
The magnetic pole of the stator forms an extending section which extends from a magnetic pole base section in a direction nearly parallel to the permanent magnet, and thereby the driving efficiency is enhanced.
That is, the width of the permanent magnet (in a direction orthogonal to the circumferential direction) is greater than the width in the same direction as that of the magnetic pole base section of the stator so as to be closer, as much as possible, to a magnetism detecting element for detecting the rotation of the rotor magnetically. Hence, the extending section is formed by extending from the magnetic pole base section of the stator in a direction nearly parallel to the permanent magnet. As a result, an opposite surface area of the magnetic pole and the permanent magnet of the stator is increased, and the driving force and the driving efficiency are increased. A similar technology is disclosed, for example, in patent document 1.
As mentioned above, in the conventional motor having the extending section formed and extended from the magnetic pole base section of the magnetic pole of the stator nearly in a direction parallel to the permanent magnet, the opposite surface area of the permanent magnet of the rotor and the magnetic pole of the stator is increased. As a result, it was generally believed that the driving force is larger and that the driving efficiency is higher.
However, according to the investigation by the present inventor, only by forming the extending section, the driving force was not always increased.
That is, following a general concept, increasing of the opposite surface area of the permanent magnet of the rotor and the magnetic pole of the stator leads to heightening of the driving force, and the extending section from the electrode of the stator should be as long as possible. However, if the extending section is increased too much, an amount of a magnetic flux from an opposite permanent magnet is increased proportionally. It leads to an occurrence of a magnetic saturation of a magnetic circuit connected to the magnetic pole of the stator. As a result, the driving force and the driving efficiency cannot be enhanced.    Patent document 1: Japanese Patent Application Unexamined Publication No. 9-285044